Most fuel cell systems are provided with a fuel cell and a battery that is charged with excess power or regenerative power from the fuel cell. In these fuel cell systems, since the fuel cell output voltage and the battery input/output voltage differ, power supply is configured to be performed by using a DC/DC converter to increase or decrease the voltage of a primary side, to which the battery is connected, to the voltage of a secondary side, to which the fuel cell is connected, or to decrease or increase the secondary side voltage to the primary side voltage.
In recent years, DC/DC converters have been developed in which several phase circuits are connected in parallel and the number of active phases can be changed. For example, Japanese laid-open patent publication No. 2006-33934 discloses a configuration in which a change of system load is predicted and the number of phases of a DC/DC converter can be changed according to the predicted load (see Patent Document 1).
Also, Japanese laid-open patent publication No. 2003-235252 suggests a power supply circuit with a master/slave-type multistage DC/DC converter, in which the input power (Pin) and output power (Pout) of the DC/DC converter are measured using a measurement device to determine the number of parallel stages of the DC/DC converter according to the output power (Pout), and the conversion efficiency (Pin/Pout) of the DC/DC converter according to increments in a commanded output voltage is calculated so as to determine the commanded output voltage that can maximize the efficiency (see Patent Document 2).
As a suggestion regarding a multiphase DC/DC converter itself, for example, Japanese laid-open patent publication No. 2006-311776 suggests a multiphase DC/DC converter that can achieve cost reduction and a long product lifetime (see Patent Document 3).    Patent Document 1: Japanese laid-open patent publication No. 2006-33934    Patent Document 2: Japanese laid-open patent publication No. 2003-235252    Patent Document 3: Japanese laid-open patent publication No. 2006-311776